PL/Perl can be used to write trigger functions. In a trigger
function, the hash reference $_TD
contains information about the current trigger event. $_TD is a global variable, which gets a separate
local value for each invocation of the trigger. The fields of the
$_TD hash reference are:

$_TD->{new}{foo}

NEW value of column foo

$_TD->{old}{foo}

OLD value of column foo

$_TD->{name}

Name of the trigger being called

$_TD->{event}

Trigger event: INSERT,
UPDATE, DELETE, or UNKNOWN

$_TD->{when}

When the trigger was called: BEFORE, AFTER, or
UNKNOWN

$_TD->{level}

The trigger level: ROW,
STATEMENT, or UNKNOWN

$_TD->{relid}

OID of the table on which the trigger fired

$_TD->{table_name}

Name of the table on which the trigger fired

$_TD->{relname}

Name of the table on which the trigger fired. This has
been deprecated, and could be removed in a future release.
Please use $_TD->{table_name} instead.

$_TD->{table_schema}

Name of the schema in which the table on which the
trigger fired, is

$_TD->{argc}

Number of arguments of the trigger function

@{$_TD->{args}}

Arguments of the trigger function. Does not exist if
$_TD->{argc} is 0.

Triggers can return one of the following:

return;

Execute the statement

"SKIP"

Don't execute the statement

"MODIFY"

Indicates that the NEW row was
modified by the trigger function

Here is an example of a trigger function, illustrating some of
the above: